1. Technical Field
This application relates to wind turbine systems and control techniques in general, and to a stand-alone wind turbine system, apparatus, and method suitable for operating the same, in particular.
2. Description of the Related Art
Renewable energy sources including wind power offer a feasible solution to distributed power generation for isolated communities where utility grids are not available. In such cases, stand-alone wind energy systems (i.e., systems not connected to the utility grid) can be considered as an effective way to provide continuous power to electrical loads.
To date, no stand-alone wind energy system is available on the market as an “off-the-shelf” product. Instead, the various components must be combined; each with their own dedicated power electronic components and controllers. Custom assembling of all discrete components offers only a limited scope for improvement or for adding new features. As well, potential problems can arise when trying to coordinate the various components in a way that does not affect reliability of the entire system.
One of the most promising applications of renewable energy generation lies in the development of power supply systems for remote communities that lack an economically feasible means of connecting to the main electrical grid. For isolated settlements located far from a utility grid, one practical approach to self-sufficient power generation involves using a wind turbine with battery storage to create a stand-alone system. If wind conditions are favourable, these stand-alone wind energy systems usually can provide communities with electricity at the lowest cost.
Small, stand-alone systems capable of producing up to a few kilowatts generally use batteries and do not have diesel-engine-driven gensets. In terms of cost per kilowatt-hour, small gensets are more expensive to buy and operate than are larger machines. Batteries therefore tend to be a more cost-effective energy storage solution for small systems. Wind and photovoltaics are often combined, because they complement each other on both a daily and seasonal basis. The wind usually blows when the sun does not shine, and vice versa.
Of particular interest are small-scale wind-battery integrated systems. Prior art configurations for wind energy systems have evolved from the configurations of several pre-existing commercial and prototype wind turbine systems. Differences in system topology and operation of these prior-art systems are summarized below.
The simplest configuration, shown in FIG. 1, is used in a domestic electrical heating system supplied by Proven Engineering Products Ltd. in the United Kingdom. The system converts wind energy directly to heat in electric water heaters. The wind turbine (WT) is operated in a variable speed mode. The direct current (dc) load consists of a set of resistors connected directly to a diode bridge rectifier. A load controller is used to obtain a better match between the wind turbine and the load. The controller closes the switch if the output dc voltage reaches a given set point, and it opens the switch if the voltage falls below this value. A hysteresis band may be included. The controller could be omitted, but its absence would dramatically reduce the total energy capture at lower wind speeds.
Stand-alone wind energy systems often include batteries, because the available wind does not always produce the required quantities of power. If wind power exceeds the load demand, the surplus can be stored in the batteries. One such system is illustrated in the configuration of FIG. 2 and was manufactured at a rated power of 4 kW by Fortis Wind Energy in the Netherlands. The system originally was designed to power radio stations in mountainous regions. Its batteries are protected against high voltage and overcharging by a charge controller, which simply disconnects the wind turbine from the batteries. Because the wind turbine can run unloaded, it may experience high open-circuit voltage and noise. The wind turbine thus requires some mechanical means of limiting its speed.
Another option for diverting surplus power is to use a dump load. The configuration shown in FIG. 3 incorporates a dump load control and was tested by ECN Wind Energy. Unlike grid-connected wind turbine systems, stand-alone systems usually include dump loads and batteries to maintain the power equilibrium between generators and loads. When the generated power is required by neither the batteries nor the load, the system diverts the power to a dump load, thus preventing the turbine from operating at a high open-circuit voltage. The wind turbine also may continue charging the batteries at a reduced voltage level, depending on the implementation of the charge control.
Existing renewable power technology relies on the combination of several discrete, commercially available components, all with their own dedicated power electronics and controllers obtained from different manufacturers. Each component operates independently without seriously disrupting the operation of the others. On the other hand, such systems rely on several controllers rather than one master controller.
Separate controller components such as battery chargers, inverters, and dump load controllers can be obtained “off-the-shelf”; however, an integrated controller that includes all of the above wind turbine control functions is not readily available. Some wind turbines may have built-in charge control features that divert their own excess power and allow it to dissipate as heat through the wind turbine housing. In most turbine systems, however, the charge controller is an external unit; and although the basic wind turbine package always includes DC rectifiers, it does not necessarily include a load-diverting controller.
In latter years, integrated controllers and systems have been custom-built for a few prototype installations. One example occurs at the Atlantic Wind Test Site (AWTS) in Prince Edward Island, Canada. AWTS has been involved in the research and development of integrated controls for wind-diesel systems (systems that combine wind turbines and diesel gensets). The controls enable wind power systems to be optimally integrated into diesel-powered generating systems. These systems are typically suitable for generating 100 kW to 1 MW, and include a number of discrete components like wind turbines, diesel gen-sets, and energy storage, which are combined together and controlled by a central computerized controller for energy dispatch. However, these systems may not be suitable to be adapted to operate at around or below 10 kW as a stand-alone system.
Lead-acid batteries used in wind energy systems have different performance characteristics than do batteries employed in more traditional applications. Batteries involved in wind energy generation commonly are subject to frequent deep-cycle discharge and irregular charging patterns, usually due to the inconsistent nature of wind speed, poor charging control, and daily load changes. Such effects are potentially damaging to the batteries. To maximize the life span of batteries, it is necessary to use the manufacturer's recommended regulation values, to follow an appropriate system design, and to practise effective charging control.
Recent advances in high-performance static complimentary metal-oxide-semiconductor (CMOS) technology have led to the development of modern digital signal processing (DSP) microcontrollers such as Texas Instruments™ TI TMS320F/C240. DSPs are now employed in the realization of sophisticated control algorithms and real-time system monitoring. In this capacity, they can play a vital role in the design of an integrated control platform that in turn will result in cost-effective and highly reliable wind energy systems.
For at least these reasons, a pressing need exists for a single power electronics package and controller that would enable all components of a wind turbine-battery-dump load stand-alone system to operate as an integrated unit. Such a unified package would eliminate duplication, achieve gains in system efficiency and robustness, and reduce overall system costs.